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Spinel Mineral Facts:

Chemical Formula: MgAl₂O₄
The magnesium may be, in part, replaced by ferrous iron, zinc or manganese and the aluminum by ferric iron and chromium.

Colors: Nearly all shades.
Color various, red, lavender, blue, green, brown, black, sometimes almost white. Streak white.

Hardness: 8.0

Density: 3.5 to 3.6

Cleavage: None - it has a conchoidal fracture, but no distinct cleavage.

Crystallography: Isometric
Habit strongly octahedral.  Often twinned on a face of the octahedron. Structure. Usually crystallized.

Luster:. Vitreous, glassy luster.  Usually translucent to opaque, at times clear and transparent.

Optics: (Refractive Index):  n = 1.718

Composition, Structure and Associated Minerals:
 A common metamorphic mineral, spinel occurs in metamorphosed limestones and crystalline schists, gneisses, and serpentines. Also found in contact metamorphosed crystalline limestones, and other rocks poor in silica where it is associated with calcite. Spinel has been described as an alteration product of corundum and garnet. It is also a primary component of igneous rocks and a product of metamorphism in rocks rich in magnesium. Found frequently as rolled pebbles in stream sands, where it has been preserved on account of its hardness and chemical durability.

Identification and Diagnostics
When black, resembles magnetite, but is not magnetic. Resembles garnet, but is not fusible. It is distinguished from pale-colored garnet by its blowpipe reactions, especially its infusibility, and its failure to respond to the test for silica. Resembles zircon, but is harder. Therefore, form, hardness, and infusibility characteristic. Spinel is easily distinguished from most other minerals by its hardness (8), its octahedral crystals and vitreous luster. Infusible. The finely powdered mineral dissolves completely B. B. in the salt of phosphorus bead (proving the absence of silica).

Occurrence, Localities and Origins:
Gem quality spinels are found most often in gravel deposits, often associated with the corundum (ruby or sapphire), in the sands of Sri Lanka, Siam, Upper Burma, Australia and Brazil. Ordinary spinel is found in various localities in New York, New Jersey and Massachusetts.

Use. When transparent and finely colored is used as a gem. Most desirable when red in color and known as the spinel ruby, balas ruby, etc. Its color is believed to be due to the presence of chromium oxide. Spinels also come in stones of a wide variety of colors. The stones usually are comparatively inexpensive, although their value has been increasing in recent years. Precious spinel is transparent and colorless or some light shade of red, violet, pink blue or green. The bright red variety is known as ruby spinel is easily distinguished from genuine ruby by the fact that it is not doubly refracting and not pleochroic. The best ruby spinels come from Burma and Sri Lanka, where they occur loose in sand associated with zircon, sapphire, garnet, etc. Gem spinels are also mined in Tanzania in East Africa.

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For More information about Spinel as a gem, see:  
SPINEL - The Bargain of the Gem World

Nevada Outback Gems